WASHINGTON --Senate Democrats thwarted a Republican effort to ban late-term abortions on Tuesday as GOP leaders strained to avoid a government shutdown in eight days over the dispute - all against a tangled backdrop of presidential politics.

Up next, in the first of a series of choreographed steps, Senate Majority Leader Mitch McConnell, R-Ky., set up a showdown vote for Thursday on stopgap legislation that would keep the government operating through Dec. 11.

But it would also block Planned Parenthood's federal funds for a year, and Democrats are expected to block that measure, too, setting up subsequent votes on must-pass bills to keep the government open free of the dispute over Planned Parenthood and abortion.

Abortion politics is roiling Congress and the White House campaign as well. A number of Republicans, outraged over Planned Parenthood's procurement of fetal tissue for scientific research, are demanding definitive action from GOP leaders.

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"If Senate Republicans cannot defund Planned Parenthood right now, there is no point in calling them Republicans," Louisiana Gov. Bobby Jindal, a candidate for the GOP nomination, tweeted last week.

President Barack Obama and congressional Democrats stand as the chief obstacles, with Democrats repeatedly blocking any legislation that undermines abortion rights.

"I just don't think that there are 60 votes in the Senate for that approach, which will then say to the House that we really need a clean (funding bill) if we're going to avoid a shutdown," said moderate GOP Sen. Susan Collins of Maine.

On Tuesday, Senate Democrats blocked a GOP measure to prohibit most late-term abortions. The Senate voted 54-42 to move ahead on the legislation, but that fell six votes short of the 60 needed to crack a filibuster mostly led by Democrats.

Tuesday's vote was the second time since this summer's release of videos involving Planned Parenthood that Senate Democrats have derailed an abortion-related drive by the GOP. It was held less than 48 hours before a first-ever papal address to Congress by Pope Francis, who leads a Roman Catholic Church that rejects abortion.

Some Republicans were unwilling to back down in the face of the Democratic opposition.

"We should stand for our principles, and our principles should not be surrendering to the Democrats," another presidential candidate, Sen. Ted Cruz of Texas, said Tuesday.

But some other Republicans insisted that an abortion fight that leads to a government shutdown would make no sense.

"I'm tired of the people on my side of the aisle who have been pushing this strategy, even though they know they don't have the votes," said Sen. Kelly Ayotte, R-N.H., up for re-election in a state Obama carried twice. "Therefore, they can't answer the question, `What's the endgame for success here?'"

Ultimately, McConnell's moves appeared aimed at delivering a temporary government-wide funding bill to the House, where abortion politics seems to have GOP leaders flummoxed.

GOP leaders in the House have staged several votes on anti-abortion legislation, but the moves haven't satisfied a handful of GOP hardliners who are insisting that the must-pass budget measure include language stripping taxpayer funding for Planned Parenthood.

McConnell has promised that a federal shutdown - which Republican leaders fear that voters would blame on the GOP - will not happen.

The showdown is reminiscent of a failed Cruz-led attempt two years ago to use a must-pass stopgap measure to try to block implementation of the health care law. That led to a 16-day partial shutdown that GOP leaders are keen to avoid this time, especially as the presidential election draws closer.

Hanging over it all is the weakened political standing of House Speaker John Boehner, R-Ohio, who is under fire from some tea party conservatives who say he is not tough enough in battling Obama. Some Republicans, including House Majority Leader Kevin McCarthy of California and New Jersey Gov. Chris Christie have called on Senate Republicans to change Senate rules to make it easier to move legislation past Democratic filibusters.

"We appreciate all the good advice we're getting from members of the House of Representatives and candidates for president about how to run the Senate," McConnell said icily. "That will obviously be a decision we make ourselves."

Another issue, little noticed so far, is that delivery of food stamp benefits to the poor could be cut off next month. That's a change from shutdowns in 2013 and 1995.

Abortion foes say videos show Planned Parenthood has violated federal prohibitions against profiting from fetal tissue sales or changing some abortion procedures to maximize the harvesting of fetus organs. Planned Parenthood says it's broken no laws and says the videos were manipulated to distort the recorded conversations.

In Tuesday's debate, McConnell described human features visible in fetal sonograms and said scientists say that fetuses can feel pain 20 weeks into development. Democrats have noted that the American Congress of Obstetricians and Gynecologists has said fetal pain is unlikely until a pregnancy's third trimester. That begins several weeks after the 20-week mark.

The Republican bill would set criminal penalties of up to five years in prison for doctors who perform most abortions 20 weeks or more into pregnancy. The House approved the legislation in May.

About 1 percent of reported abortions involve pregnancies of 20 weeks or more, according to the federal Centers for Disease Control and Prevention.