Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellFEC flags McConnell campaign over suspected accounting errors Poll: 59 percent think president elected in November should name next Supreme Court justice Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ky.) on Tuesday will become the longest-serving Senate Republican leader in history, surpassing former Sen. Bob Dole (Kan.), a testament to his canny political skills and loyalty within the GOP conference.

Tuesday will be McConnell’s 4,179th day as Senate Republican leader, according to the Senate Historical Office, a position he ascended to on Jan. 3, 2007, after Republicans lost control of both chambers of Congress.

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Senate Republican Whip John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (Texas) is expected to mark McConnell’s record-setting tenure with a floor speech Tuesday.

As the Senate Republican leader, McConnell has kept a steady hand on the party through two political revolutions within the GOP: the emergence of the Tea Party in 2009 and 2010 and the election of President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE, a stunning upset that even surprised many Republicans.

McConnell helped his party come back from a disastrous defeat in the 2008 elections to regain power across the government, including control of the White House, Senate, House and 33 governorships.

After becoming the Senate majority leader in January 2015, McConnell and his caucus blocked former President Obama’s nominee to succeed the late Antonin Scalia on the Supreme Court, preserving a conservative majority. The move enraged Democrats but may end up being McConnell’s most lasting legacy given the confirmation of Trump’s pick for the court, Justice Neil Gorsuch.

He was also able to keep his caucus unified in the first year of Obama’s presidency, when Democrats looked like they might steamroll the GOP. Democrats had a president in the White House, a 60-vote supermajority in the Senate, 263 seats in the House and 32 governorships.

Yet only three Republican senators voted for Obama’s $787 billion fiscal stimulus package: Sen. Susan Collins Susan Margaret CollinsClub for Growth to spend million in ads for Trump Supreme Court nominee Maryland's GOP governor says Republicans shouldn't rush SCOTUS vote before election The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November MORE (Maine) and former Sens. Olympia Snowe (Maine) and Arlen Specter (Pa.). Specter later switched parties.

McConnell led an all-out fight against Obama’s signature health-care reform law, which not a single Senate Republican backed.

Former aides and allies of McConnell, who is often a target of derision by grass-roots conservatives, say he helped rekindle the GOP’s fighting spirit and that he does not always get the respect he deserves.

“What sticks out to me is the difference in where the Republican Party as a whole is from the time he started that job until now,” said Josh Holmes, McConnell’s former chief of staff.

“Trying to unify and galvanize opposition to a new liberal agenda with all the wind at its back was as difficult as it could get,” he added.

Democratic critics say the hardball tactics McConnell used to unify Republicans and thwart Obama’s agenda contributed to a souring of bipartisan relations.

Jim Manley, a longtime Senate aide and former spokesman for former Senate Majority Leader Harry Reid Harry Mason ReidThe Supreme Court vacancy — yet another congressional food fight Trump seeks to turn around campaign with Supreme Court fight On The Trail: Battle over Ginsburg replacement threatens to break Senate MORE (D-Nev.), who clashed with McConnell often, said McConnell has been a much more partisan leader than Dole.

“Sen. Dole believed in the art of compromise and was more than willing to reach across the aisle to work with Democrats. As far as I’m concerned, Sen. McConnell has never been willing to do that except under the most extreme circumstances,” Manley said.

He argued that the number of partisan filibusters jumped dramatically under McConnell’s leadership.

There were 54 cloture votes to end dilatory debate in 2005 and 2006.

That number jumped to 112 in 2007 and 2008 and 91 in 2009 and 2010, according to the Senate’s website.

“He’s the one responsible for breaking the Senate once and for all,” Manley said.

McConnell in recent weeks has been praising the current Congress as one of the most productive for conservative causes, seeking to keep the Senate majority in the GOP’s hands.

“This is quite an opportunity we’ve been given by the American people, a great opportunity,” he said Friday at an event sponsored by the Faith and Freedom Coalition.

He noted that Republicans have controlled the White House and both chambers of Congress for about 20 years in the last 100.

“We’ve tried to maximize this opportunity,” he said.

McConnell was cautious in embracing Trump during the 2016 presidential race and even admonished him at times for going off script with such zeal, but he has since become a crucial ally in passing the president’s agenda and confirming his nominees.

“I’m now in my third decade in the Senate. This has been the best period, the best period right of center over the last 17 months, in the time that I’ve been here. It’s been a period of extraordinary accomplishment,” he told The Hill in an interview last month.

“We think we have made a very significant difference for the country in measurable ways,” he added.

Beyond the conservative record of this Congress, McConnell’s allies point to his leadership during three momentous times in recent history: the 2008 financial collapse, the 2011 debt-limit standoff and the 2012 fiscal cliff, when the Bush-era tax cuts were due to expire and spending cuts due to take effect.

McConnell supported the controversial 2008 Troubled Asset Relief Program — which supporters view as saving the economy from collapse.

It drew harsh condemnation from conservatives, who labeled it a Wall Street bailout. McConnell called it “one of the finest moments in the history of the Senate” after the Senate saved the package days after it was defeated in the House — which then reversed course and passed it, sending it to President George W. Bush for his signature.

McConnell negotiated a way out of the 2011 debt standoff between the GOP and Obama by hatching a proposal to give the president full authority to raise the debt limit and limiting Congress to voting on a resolution of disapproval, thereby sparing GOP colleagues from taking an unpopular vote.

He helped avoid another stock market calamity at the end of 2012 when he and then-Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE struck a deal to make permanent the Bush tax cuts for individuals earning up to $400,000 and families earning up to $450,000 annually.

“This shouldn’t be the model for how to do things around here,” McConnell said at the time. “But I think we can say we’ve done some good for the country.”