(Updates with details)

By Thomas Ferraro and Richard Cowan

WASHINGTON, Jan 6 (Reuters) - Democrats sealed their increased control of the U.S. Congress on Tuesday with the swearing in of newly elected members expected to help enact President-elect Barack Obama's call for a massive economic stimulus package.

Gavels sounded at about noon EST (1700 GMT), opening the Senate and House of Representatives a day after Obama visited Capitol Hill to press for quick action on a $750 billion package of tax cuts and new spending designed to stem a deepening recession.

Obama takes office on Jan. 20, giving Democrats control of the White House, the House and Senate for the first time in 14 years.

Vice President Dick Cheney, in his final days as Senate president, swore in the newly elected senators as one of the first orders of business.

Despite such challenges as the economy, two wars and an estimated 46 million Americans without health insurance, the opening day of Congress was something of a victory party for the Democrats.

Yet the celebration was muted by a flap over a pair of seats that remained empty in the 100-member Senate.

Senate Democrats refused to swear in, at least for now, a fellow party member, Roland Burris of Illinois. He was appointed to fill the seat vacated by Obama by a governor engulfed in scandal, but the secretary of the Senate rejected his credentials as incomplete amid controversy over the appropriateness of the appointment.

And Senate Democratic Leader Harry Reid yielded to Republican pressure and put off an effort to seat comic-turned-politician Al Franken of Minnesota, also a Democrat, who is facing legal challenges of his razor-thin election victory.

If both had been sworn in, the Democrats' majority in the 100-member Senate would swell to 59 -- their biggest margin in 30 years and just one short of the needed 60 votes to overcome Republican roadblocks.

But with the possibility of court battles, back-room negotiations and procedural wrangling, it could take months to fill the two seats.

(Reporting by Thomas Ferraro and Richard Cowan, editing by David Wiessler)

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