The Kentucky Republican could go down as one of the most influential Senate majority leaders in history. Why? Here's everything you need to know:

What is McConnell's strength?
As the Senate majority leader, Mitch McConnell controls the calendar for the upper chamber, giving him the ability to block legislation by simply refusing to schedule a vote. The Kentucky Republican has combined that ­authority — and intricate understanding of Senate rules and ­procedure — with ruthless partisanship to ensure that Republican-backed legislation and nominees are fast-tracked while all Democratic priorities are blocked. Democrats complain that McConnell has turned the Senate into a "legislative graveyard" by not allowing bills passed by the House of Representatives to even be considered on the Senate floor. During the Trump presidency, McConnell has focused on confirming judges to reshape the country through the courts. So far, the Senate has confirmed more than 150 lifetime judges appointed by President Trump. Conservative legal activist Leonard Leo of the Federalist Society says McConnell's impact on the courts will be felt for decades to come, describing him as "the most consequential majority leader, certainly, in modern history."

How did he get his start?
McConnell began his political career in the GOP's once influential liberal wing. As a college student, he wrote an op-ed against segregationist politics, arguing that a "strict interpretation" of the Constitution was "inherently evil" if it was used to deny basic civil rights. He even came out in favor of "truly effective campaign finance reform." In 1977, he won election as judge-executive for Jefferson County, which encompasses Louisville, campaigning in favor of collective bargaining rights for public workers, and for abortion rights. Although not personally charismatic, McConnell showed fundraising savvy and keen political instincts. For his 1984 Senate run, he hired high-profile Republican political consultant Roger Ailes, the future founder of Fox News. Ailes created a devastatingly effective ad depicting bloodhounds searching for Democratic incumbent Walter "Dee" Huddleston, exaggerating his record of missed Senate votes. McConnell won the election by 5,000 votes, about 1 percent of the total, becoming the first Republican elected statewide since 1968.

What does he believe in?
The Republican Party and winning elections. As the GOP moved right under President Ronald Reagan, so did McConnell. Political scientists tracking McConnell's career have found that he has become more conservative with every session of Congress. His earlier support for campaign spending caps was transformed into fierce opposition to campaign finance reform, and he won the gratitude of many senators by taking the heat for blocking bills to rein in spending on elections. McConnell embraced his villain status. When pundits labeled him "Darth Vader" for opposing the McCain-Feingold reform bill in the early 2000s, he started carrying a toy lightsaber. McConnell admitted that he never would have won his first race for Senate "if there had been a limit on the amount of money I could raise and spend."

Why is he so effective?
During his 34-year career in the Senate, McConnell has learned to work the rules to his and his party's advantage, both in the majority and out of it. When McConnell took over leadership of the Senate Republicans in 2007, he quickly proved to be a cunning minority leader. Huddling with his shell-shocked GOP caucus in the aftermath of President Obama's overwhelming victory in 2008, McConnell argued for a strategy of near-total obstruction to dim enthusiasm for Obama, who had campaigned as the man who could transcend Washington's partisan divide. McConnell used the filibuster — a tactic to block action — to impede or slow-walk almost everything Democrats tried to achieve. During the 2013-14 term, Democrats called 252 cloture votes, which are motions used to prevent or end filibusters — double the previous record.

Did McConnell's strategy work?
Arguably, yes. Disgusted by Washington's gridlock and disappointed in Obama, voters punished the Democrats in power, handing the GOP the House in 2010 and the Senate in 2014. Meanwhile, McConnell blocked as many of Obama's judicial appointments as possible. When Obama left office, he'd been unable to fill 88 district- and 17 circuit-court seat ­vacancies — plus the biggest vacancy of all. After Justice Antonin Scalia died in February 2016, McConnell infamously refused to hold a vote for Obama's Supreme Court nominee, Merrick Garland, holding the seat open for 10 months for the next president to fill. That vacancy proved critical in convincing many reluctant Republicans to support Donald Trump in a close election. With Trump in office, McConnell abolished the filibuster for Supreme Court nominations, allowing Republicans to confirm Neil Gorsuch and Brett Kavanaugh with slim majorities. Although initially skeptical of Trump, he has long since made his peace with the president. "To expect Republican elected officials not to try to achieve as much as they possibly can out of pique over presidential behavior," he says, "is nonsense."

How he wins elections
Despite being the longest-serving Republican Senate leader, Mitch McConnell has never been beloved either in his home state or by conservatives. Activists on the Right have always been suspicious of McConnell's transactional nature and angered by his willingness to cut limited deals with Democrats to keep the government running. He was even booed at the 2016 Republican National Convention. McConnell's 36 percent approval rating in Kentucky makes him one of the most unpopular senators in the country, and this year he is likely to face a well-funded challenge from Democrat Amy McGrath, a former Marine fighter pilot. But McConnell has built up a massive campaign war chest, raising $10.6 million so far, more than any Republican ­senator — money he's almost certain to pour into negative ads. Kentucky insiders have seen this play out before. McConnell has never lost any of the nine elections he's run in. "All those polls you see now where he has a low approval rating? That's because he doesn't have a warm-and-fuzzy personality," says veteran Kentucky political reporter Al Cross. "In those polls he's running against himself. When you match him up against somebody, he's pretty good at driving them down to his level."