By helping Democrats pass a landmark $750 billion climate, tax and health care bill, Sens. Joe Manchin of West Virginia and Kyrsten Sinema of Arizona have complicated their own re-election fortunes for 2024.

Why it matters: While Democrats have a credible chance to hold onto a narrow Senate majority this year, they're staring at a rough political map in two years, when both of those seats are up — and when the party will also be forced to defend seats in red states like Montana and Ohio.

• The fortunes of battle-tested Democrats like Sinema and Manchin will determine the party's ability to hold a long-term Senate majority.

What we're watching: Manchin, who enjoyed bipartisan approval as a check against the Biden White House, is now being tagged by Republicans as a supporter of Senate Democratic Leader Chuck Schumer's liberal agenda.

• He's already getting attacked on television by a potential GOP challenger, Rep. Alex Mooney, for "destroying" West Virginia's coal industry.

Sinema, who already faced distrust from progressives, ultimately ensured the bill's passage.

• But she held out her support in order to protect a tax break for private equity executives and hedge fund managers, aggravating both progressive activists and even moderate former Treasury Secretary Larry Summers.

Zoom in: Supporting any legislation boosting clean energy won't be an easy sell in coal-producing West Virginia. And Sinema's vote, in tandem with her private equity push, opens up new lines of attack from her political rivals on both the left and right.

• Manchin secured a key concession easing permitting for fossil fuel projects, but it's unlikely to be enough to persuade Trump-loving voters that the bill favors their interests. "He's up against the entire Democratic machine and Republican machine, who are both calling it a climate bill. That's not helpful," said one source familiar with Manchin's thinking.
• His approval rating soared after he scuttled Biden's original multitrillion-dollar social spending bill, jumping 17 points — from 40% to 57% — in the first quarter of this year. He's now expected to lose some of that bipartisan support after negotiating the spending deal.

Be smart: Sinema's vulnerabilities are more complex.

• At a time when she's winning over Republicans and independents back home, her support for the partisan legislation handed a future GOP challenger a ready-made attack line: She's more interested in protecting Wall Street than Main Street.
• At the same time, unlike Manchin, Sinema also has to worry about the possibility of a primary challenge from her left. Arizona Rep. Ruben Gallego is already teasing a possible primary challenge against the maverick Democrat in 2024.

Between the lines: Sinema was concerned that penalizing private equity firms would adversely affect the small businesses they support — an issue that every other Senate Democrat facing a tough race in 2022 also agreed with. And she also used her leverage to guarantee additional funding for drought resilience, which is a major issue in arid Arizona.

• "At a time of record inflation, rising interest rates and slowing economic growth, disincentivizing investments in Arizona businesses would hurt Arizona's economy and ability to create jobs," Sinema spokesman Hannah Hurley told Axios.

The bottom line: With inflation dominating voter concerns for the midterms, it's unlikely the climate, tax and health care bill will have a significant political impact in 2022.

• But if Manchin's and Sinema's roles in this party-line bill were to weaken two of the upper chamber's most independent members, it could have major ramifications for the Democratic Party in 2024.