Congress reached its last day to end the stalemate over a possible government shutdown.

WASHINGTON — As Americans awoke to find Sen. Ted Cruz (R-Texas) still conducting his all-night talkathon to stop President Obama's healthcare law on Wednesday, some also found a pleading email in their in-boxes.

"We can win this fight, but we must do our part," read the message from the Senate Conservatives Fund with the subject line "Still Standing." The group specializes in helping conservative Senate candidates — sometimes in campaigns against other Republicans.

"If you want to help us continue to put pressure on wavering Republicans, please donate $5 or more," the message said. "We are doing everything we can to win this critical fight."

The last week of standoffs and stalemates in Washington won't help Congress' dismal approval ratings. And the likelihood that most government programs will begin shutting down Tuesday already has started disrupting the lives of millions of federal government workers, contractors and their families.

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But for one group — fundraisers who collect cash for members of Congress and those hoping to join the club — the shutdown threat is a windfall.

Republicans have insisted that any measure to fund government agencies for the new budget year, which begins Tuesday, must be tied to blocking Obama's healthcare law. As that fight escalated, organizations on both sides flooded supporters with fundraising pitches tied to pledges by members of the House and Senate to stand firm for "defunding Obamacare" or to "resist Republican blackmail."

Until recent years, members of Congress depended heavily on contributions ginned up by lobbyists, trade associations and other groups with interests in specific pieces of legislation. Those groups tended toward legislative deal-making, often at the expense of broader ideologies.

They still exist, but their clout has been eclipsed by groups that thrive on ideologically polarizing issues. Social networking technology has made it easier to appeal to large groups of voters outside Washington. Court decisions and changes in election laws have wiped out limits on how much groups can spend, dramatically increasing the influence of outside organizations on political campaigns and legislative priorities.

The groups, in turn, feed the polarization of Congress, pushing members toward the extremes and sometimes threatening to fund primary challenges against those who consider compromise. Political scientists who study Congress say the degree of partisan polarization in the House and Senate is greater now than at any time since the late 19th century.

"Demonizing the opposition is central to the process" of raising campaign money in the current environment, said Steven S. Smith, political science professor at Washington University in St. Louis. "The more important the development in Washington, the easier it is to scare potential donors into handing over the cash." Spending deadlines, he said, become "natural targets."

Traditional party leaders have seen their ability to orchestrate strategy upended as their dominance over campaign money has waned. The shift in power has been particularly dramatic on the Republican side because of the tea party insurgency, which has threatened many incumbents, and because the party lacks the countervailing power center provided by the White House. Because of how active groups on the right have become, many Republicans have become "less compliant party members," Smith said.

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The U.S. Chamber of Commerce and other business groups that have long enjoyed influence among Republicans have appealed to GOP members to avoid a confrontation that could lead to a shutdown of government services. But they have had little ability to match more dramatic appeals on the right.

"This isn't the first time the Establishment betrayed you," read an email from FreedomWorks, a tea party umbrella organization, shortly after the Senate voted to reject one of Cruz's procedural moves. "That's why you and I must replace the Obama-Republicans with real conservatives…. If we're going to replace them, if we're going to stop Obama, we need you to donate $50, $35, $15 or more right now!"

Democratic groups have launched their own high-decibel appeals.

"Don't let the GOP take us back to the 1950s! Are you with us?" tweeted Emily's List, the organization that raises money to elect pro-choice female candidates. Their appeal focused on a provision House Republicans put into the latest version of the budget bill. It would enable employers to exclude contraceptive coverage in company insurance plans if they claim they have moral objections to birth control.

Fundraising even has started to influence such basic parts of the process as when votes take place. That burst into public view Thursday, when one Republican senator suggested in a floor speech that Cruz and his allies were playing for the cameras — and, by implication, potential donors — when they refused to agree to a vote that night and insisted it take place Friday.

"I am understanding the reason we are waiting is the senators have sent out press releases and emails, and they want everybody to be able to watch," said Sen. Bob Corker (R-Tenn.). "It does not seem to me that is in our nation's interest, nor is it, candidly, in the interests of those who want to see good policy on the conservative side come out."

Appeals to ideological purity have boxed in party leaders. Over the weekend, House Speaker John A. Boehner (R-Ohio) had little alternative but to double down on the strategy pushed by Cruz and his tea-party-backed allies, even though their approach has yielded no results so far, and many Republican strategists consider it doomed.