If the past is any indication — and it usually is — Republicans should tread carefully with their latest Benghazi investigation. Unless they actually find a truly smoking gun — and so far countless probes have failed to do so — 20 years' worth of data suggests the GOP will wind up shooting itself in the foot.

The data comes from Gallup. The polling firm looked at the popularity of both major parties going back to the early 1990s, and drew a clear connection between GOP escapades designed to fire up its base and the party's sinking reputation with the American people as a whole. There have been two Republican low points over the last 20 years — a 31 percent approval in late 1998, just as the GOP led the fight to impeach President Bill Clinton, and a 28 percent approval last fall, when conservative members of the House shut down the federal government in a quixotic attempt to kill ObamaCare.

The data suggests that these hyperpartisan, proactive events wound up damaging the Republicans in the eyes of the American people. Both did, however, help the GOP fire up its base and raise money, not inconsequential things. Getting your base to write checks and show up at the polls is how you win elections.

We're seeing this again now. The GOP-led Benghazi panel is galvanizing the Republican base, and is being attached to fundraising pleas, even as most Americans shrug.

Meantime, Democrats have problems of their own. While the GOP has seen its favorability rating plummet 19 percentage points over the last two decades, the Dems have seen their favorability rating sink a still-worrisome 10 percentage points.

Yep, Democrats aren't exactly basking in popularity, either. Their approval peaked at 61 percent way back in 2000, in the aftermath of the Clinton impeachment. It had a small bump in 2008 when Obama was elected. But Democrats — and the president himself — have slid ever since, hovering in the low-to-mid 40s since 2010.

But here's the thing: Overall popularity isn't all it takes to win elections. It's also about turnout, as well as intradistrict and intrastate popularity. This chart tells the story:

For an entire generation, there hasn't been a single presidential election year in which the Republican Party has had a higher approval rating than the Democratic Party. With the exception of 2004, Democrats have won the popular vote in every single presidential election during this period — 1996, 2000, 2008, 2012. But in midterm years, Republicans tend to do well. In 2002, they upped their House majority and seized the Senate. In 2010, a wave election gave them control of the House, and a pickup of six Senate seats. The only midterm election during this period in which Republicans fared poorly was 2006, when rising discontent with George W. Bush helped Democrats take control of Congress for the first time since 1994.

Thanks to gerrymandering fueled by the GOP's gubernatorial and statehouse sweeps in 2010, today's congressional districts are drawn extremely favorably for House Republicans. In many districts, GOP representatives can honestly say shutting down the government over ObamaCare is exactly what their constituents want them to do, even as such actions crater the party's popularity nationally.

So even though the GOP's favorability rating is in the tank, you can probably expect Republican gains in the House in November — four to six seats, perhaps, and anywhere from three to eight in the Senate. The GOP needs to pick up six seats in the Senate to win control, and it's quite possible they'll do it, given that seven Senate Democrats are defending seats in states won by Mitt Romney in 2012.

Here's what will happen if Republicans gain in the House and take back the Senate in November: They'll crow about how they're on the right track, how most Americans support their policies, and what a repudiation of Barack Obama it was. But what it really means is that they did a better job of getting their base — largely white, older, and male — to the polls in key districts and key states. It won't say diddly squat about 2016.