When the history of the 2010s is written, Senate Majority Leader Mitch McConnell (R-Ky.) will be remembered as the most-consequential American politician of the decade.

Not President Barack Obama, though his status as the nation's first African-American president will loom large in history. Obama's presidency would have turned out much differently if not for the opposition of McConnell — and in particular McConnell's decision to deny a Senate hearing for Merrick Garland, Obama's last Supreme Court nominee. It was a critical moment in building a conservative SCOTUS majority that could last for decades and seems positioned to transform our collective understanding of Constitutional law.

President Trump won't win this contest, either, though he will be remembered for leading the backlash against Obama's presidency, and for helping usher in an era of politics in which the notion that truth matters seems to have disappeared entirely. You can have your own opinions and your own facts, it turns out. But McConnell was smashing norms and precedents in the Senate even before Trump arrived in Washington, D.C. In the case of Trump's major accomplishments — cutting taxes and transforming the American judiciary — McConnell probably deserves the lion's share of the credit.

We Americans tend to remember historical eras through the lens of the presidency. But McConnell, more than most Senate leaders, served as a gatekeeper for what the presidents of his era have been able to accomplish — and he might be the most powerful and significant senator since Lyndon Johnson in the 1950s, and this was true despite the fact that he and his party shifted back and forth throughout the decade between opposition and majority status.

McConnell's influence was felt broadly, but particularly in three crucial areas:

"Party of No": One of McConnell's key acts this decade was actually set in motion a couple of years earlier, when Obama was elected in 2008. McConnell helped create a strategy of never cooperating with the new president, on the belief that voters would blame Obama — and not Republicans — for the resulting gridlock. "If he (Obama) was for it," former Ohio Sen. George Voinovich said of McConnell's strategy, "we had to be against it."

Michael Grunwald reported for Time:

Vice President Biden told me that during the transition, he was warned not to expect any bipartisan cooperation on major votes. "I spoke to seven different Republican Senators who said, 'Joe, I'm not going to be able to help you on anything,'" he recalled. His informants said McConnell had demanded unified resistance. "The way it was characterized to me was, 'For the next two years, we can't let you succeed in anything. That's our ticket to coming back,'" Biden said. [Time]

So Republicans became known as the "party of no." They opposed Obama's $800 billion stimulus package in the middle of the Great Recession. They refused to sign on to a universal health-care program modeled on one passed by a Republican governor, Mitt Romney, in Massachusetts.

"On just about every issue, from ObamaCare to climate to education reforms that conservatives supported until Obama embraced them, Republicans have embraced that strategy" of total opposition, Grunwald wrote for Politico in 2006. "Senate Republicans even turned routine judicial nominations into legislative ordeals, filibustering 20 of his district court judges — 17 more than had been filibustered under all of his predecessors."

Transforming the judiciary: Indeed, McConnell's singular legacy will probably be his long-term effort to give conservatives dominance of the federal judiciary. His blockade of Garland's nomination to the Supreme Court was the most famous example of that mission, but possibly not the most important. His blockade of Obama's lower-court appointments led then-Senate Majority Leader Harry Reid (D-Nev.) to eliminate the filibuster for those offices. (McConnell would later make a similar rule change to get Trump's SCOTUS nominee, Neil Gorsuch, approved by the Senate.)

After Republicans regained control of the Senate in 2014, the blockade of Obama's appointments became more pronounced. The result: Trump inherited 88 district court vacancies, along with 17 appellate court positions in need of filling. The president gets lots of publicity these days for all the judicial appointments he has made, but it was McConnell who spent the decade setting the stage.

"This is my top priority in the Senate," McConnell told conservative radio host Hugh Hewitt last year. "What I want to do is make a lasting contribution to the country. And by appointing and confirming these strict constructionists to the courts who are in their late 40s or early 50s, I believe working in conjunction with the administration, we're making a generational change in our country that will be repeated over and over and over down through the years."

The result is a court system that for the next few decades will be less friendly to abortion, LGBT, and minority rights and the regulatory state, but friendlier to gun ownership and business interests. We haven't even scratched the surface of the Constitutional law changes that are coming thanks to McConnell's efforts.

From Russia with love: If Trump needed McConnell to transform the judiciary, the reverse is also true. So McConnell's role in helping Trump get elected is both notable — and, even now, shocking.

The Obama administration in 2016 determined that Russia was attempting to interfere with the presidential election, and presented the evidence to congressional leaders. McConnell reportedly challenged the findings, "and made clear to the administration that he would consider any effort by the White House to challenge the Russians publicly an act of partisan politics." He refused to sign on to a joint statement warning the public about that interference, and Obama officials were hesitant to sound the alarm without his participation. Russia's efforts on Trump's behalf didn't start to get a full airing, then, until after the election. Would the election results have changed if the public had been more fully informed? That question is destined to be one of American history's great what-ifs.

"Our administration's interest in making sure the response was bipartisan wasn't for the sake of being bipartisan," an Obama-era official later argued. "It was necessary because we needed the buy-in from state and local election administrators (many of whom were Republican partisans and/or skeptical of federal government). Unfortunately, as is well documented, Senator McConnell was unwilling to help — only making matters worse."

McConnell spent the decade undermining one president and paving the way for another, all in support of an effort to fundamentally alter the way Constitutional law is interpreted and enforced. He is the thread that connects the shortcomings of the Obama administration to the rise of Trump, and beyond. Because of all that, he will be remembered — possibly not fondly — as the most important American politician of the decade.

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