BY Ed Stoddard

THE hot-button issue of abortion was supposed to have receded from the United States political scene, with the election of the pro-choice, moderate Barack Obama.

Instead, it is now taking centre stage, with an abortion-related amendment proving critical to President Obama's flagship health-care initiative, and which is now being called "a political bombshell that has ripped holes in both parties and set the stage for the 2010 mid-term congressional elections".

Mr Obama's bid to overhaul the US health-care system has exposed a split within his own Democratic Party over abortion. This threatens to undercut the party's gains in recent elections.

The Democrats, especially the liberal wing, have traditionally supported a woman's right to terminate a pregnancy.

But a rising chorus of moderate Democrats - some elected in traditionally Republican districts during the Democratic sweeps of Congress in 2006 and last year - are openly challenging the party's stand on the issue.

The rift came to a head this month as the US House of Representatives prepared to vote on a Bill to expand medical-insurance coverage, as part of the health-care reform bid.

Facing an internal revolt by moderates, the Democratic leadership was forced to include a measure barring federal subsidies from health policies covering abortion, to assure the Bill's passage.

The Senate leadership has excluded such language from its Bill, guaranteeing a confrontation, if it passes, between the two chambers of Congress.

Ms Jennifer Duffy, who tracks Senate races for the nonpartisan Cook Political Report and is the observer who called the House abortion amendment a "bombshell", characterises it as a suddenly vital litmus test.

Abortion-rights activists are lobbying Democrats in Congress in a bid to stop what they see as an effort to erode a woman's "right to choose".

They want the House to excise the offending passage when the next legislative stage - deliberations between both chambers - begins.

If the anti-abortion Democrats hold firm, it is not impossible to see health-care reform collapsing.

Said Ms Duffy: "It is a vote, no matter what side you are on, that will help galvanise the base of both the Democrats and their foes, the Republicans."

The abortion issue was first rekindled last summer as evangelical conservatives drummed up opposition to Mr Obama's health-care plan, charging that it would lead to government funding of abortion.

With Republicans - who have the strong support of evangelical Protestants - generally united in opposing abortion, it was the moderates within the Democratic Party who felt the heat most.

Democratic congressmen elected in districts carried by Republican presidential candidate John McCain last year were seen as particularly vulnerable and were targeted for intense anti- abortion lobbying.

For some analysts, the Democrats have become a victim of success as they gained control of Congress and expanded their majorities, partly by fielding anti- abortion candidates who won in Republican areas.

The battle underscores a key quandary facing the Democrats - how to push through the Obama agenda, including health-care reform, without jeopardising key blocs of moderate and socially conservative voters.

The tightrope is made more tricky by polls showing reduced support for abortion rights. Wooing Catholics, who make up about 25 per cent of the population, is vital for Democrats and key to Mr Obama's chances of re-election in 2012.

Though the US Catholic Church supports the Obama health-care drive, it is dead set against abortion. Lobbying by Catholic bishops was crucial to the inclusion of the anti-abortion amendment attached to the House health-care Bill.

"The Catholic bishops really want some kind of health-care reform," said Mr Matthew Wilson, a political scientist at Southern Methodist University in Dallas.

"Yet, right now, it (abortion) is the biggest immediate obstacle to getting a Bill passed."