As a Democrat from West Virginia, Manchin has thrice won election to Congress’s upper chamber in a state that, politically speaking, runs deeply red as a Republican stronghold. He’s done it the same way he won two terms as West Virginia’s governor beforehand—as a conservative, “blue dog” Democrat who, despite caucusing on the left side of the aisle, frequently aligns with Republicans on issues such abortion, gun rights, environmental regulations, and fiscal policy.

From his position at the center of the American political spectrum, Manchin has managed to hold considerable sway on Capitol Hill, where the Democratic leadership knows better than to rely on him to toe the party line. During the Trump presidency, Manchin voted in line with the former President on legislation more than 50% of the time, according to FiveThirtyEight—a mark matched among sitting Democratic senators only by Kyrsten Sinema, his fellow centrist, from Arizona.

Manchin’s propensity for bipartisanship and his nonideological brand of lawmaking have won him plenty of plaudits. They’ve also left him in good stead with a West Virginia electorate that—despite favoring Trump over Joe Biden by a 2-to-1 margin in the 2020 presidential election—continues to elect Manchin to office as a Democrat. Yet those same inclinations have often put his own caucus in a bind as far as pursuing its legislative goals, and they’re now in the spotlight as congressional Democrats look to advance President Biden’s agenda.

While Democrats’ surprise double-victory in Georgia’s Senate runoff elections last month gave the party control of both houses of Congress, their Senate majority is razor-thin. With the chamber’s 50-50 split, the Democrats gain the upper hand only via Vice President Kamala Harris’s tiebreaking vote. That means that every vote counts, and keeping Manchin and other centrist Democrats on board with the party’s legislative ambitions is crucial if Biden is to enact much of his first-term agenda.

Already Republicans have detected Manchin as a point of weakness for Democrats, a way of keeping their rivals’ narrow majority in check over the first two years of the Biden administration. Last month, Senate Minority Leader Mitch McConnell (R-Ky.) refused to strike a power-sharing deal with Majority Leader Chuck Schumer (D-N.Y.) unless Democrats assured him that they wouldn’t get rid of the legislative filibuster, a procedural rule that effectively requires a 60-vote majority to pass most bills in the Senate. McConnell eventually relented—but not without touting commitments from Manchin and Sinema that they would not vote to end the filibuster.

Now, as Biden seeks to pass a massive $1.9 trillion coronavirus relief package, the President and top congressional Democrats are finding yet again that Manchin holds the cards as far as what the final bill will look like. Even before Biden took office, Manchin expressed skepticism about another round of direct payments of up to $2,000 per person. He’s now part of a bipartisan group of senators who are lobbying the administration to scale down its proposed stimulus—particularly direct payments to higher-earning households—in favor of more “targeted” relief.

Manchin has been a vocal advocate for collaboration on the stimulus package, urging the White House to strike a filibuster-proof deal with Republicans instead of resorting to budget reconciliation measures to push the legislation through the Senate with a simple majority. While President Biden has signaled a preference for bipartisanship and met with Republican senators on Monday night, Republicans have pitched a $600 billion counterproposal that falls well short of the administration’s goals, and top Democrats have indicated that they’re prepared to move forward with budget reconciliation if necessary.

But even then, Manchin could single-handedly determine whether or not the bill fails, as budget reconciliation would require his support in order to pass the Senate with a party-line vote. As such, the White House would do well to keep him on its side—something that may be easier said than done, the way things are going.

Manchin was not amused by Vice President Harris’s appearance last week on a local West Virginia news channel—an interview in which she stressed the need for urgent economic relief for Manchin’s constituents. It was a curiously conceived media appearance that many political observers took as an attempt to pressure the senator into backing Biden’s proposal, including the more lucrative stimulus checks that Manchin has thus far balked at.

But if Manchin’s reaction is anything to go by, it may have backfired.

“I saw [the interview]; I couldn’t believe it. No one called me,” he said of Harris’s comments. “We’re going to try to find a bipartisan pathway forward—I think we need to. But we need to work together. That’s not a way of working together, what was done.”

The Biden administration will likely be eager to smooth things over with Manchin, such is his influence over proceedings. On Monday, White House press secretary Jen Psaki called Manchin “an important partner as we look to move forward on this package and, of course, all of the President’s agenda.”

Given the precarious balance of power in the Senate, that would indeed appear to be the case. Because as Manchin goes, so does the Democrats’ majority—and with it, President Biden’s ability to implement his vision for America.