Sen. Dean Heller ( R-Nev.) kisses his wife Lynne Heller in the Old Senate Chambers on Capitol Hill prior to his mock swearing ceremony. (Alex Brandon/AP)

Vice President Joe Biden visited the Capitol to administer the oath of office to Heller about 2:15 p.m.

The swearing-in does not change the Senate’s party breakdown but does give Heller -- who had announced his candidacy for the seat after Ensign’s retirement announcement, but before Ensign’s abrupt resignation this month -- a leg up in running to hold the seat in November 2012. Nevada Gov. Brian Sandoval (R) announced last month that he was appointing Heller to the seat.

As he left the Old Senate Chamber after Monday’s ceremonial swearing-in photo-op, Heller told reporters that his first official action as a U.S. senator was signing onto the balanced budget amendment proposal already co-sponsored by the 46 other Republicans in the Senate.

“The first thing that I did is that I signed on as co-sponsor to the balanced budget amendment,” Heller said. “I think that’s probably the most important issue facing this country right now.”

Heller will likely face a competitive race to hold onto the seat, and even before his swearing-in, national Democrats had been targeting his vote in favor of House Budget Chairman Paul Ryan’s (R-Wis.) 2012 budget blueprint.

The Nevada Republican reiterated Monday that he plans to vote in favor of the Ryan budget when it comes up for a vote in the Senate, but in defending the plan, he cast the House Republican budget blueprint as the lesser of two evils.

“I just look at the alternative; the alternative, of course, is to raise taxes,” Heller said. “I mean, if you take a look at the alternatives to the Ryan budget, it’s anywhere from two to 16 trillion dollars in tax increases, and rationing health care. So for that reason, I don’t want to raise taxes; I don’t want to ration health care; I don’t want to take money away from Medicare. The Ryan budget puts half-a-trillion dollars back into Medicare; that’s why I support it.”

The already-crowded field of candidates seeking Heller’s former House seat in Nevada’s second district became even more packed Monday when state Republican Party Chairman Mark Amodei entered the race, joining former Assemblywoman Sharron Angle (R) and several other candidates. Asked Monday whether he will be backing a candidate in the Sept. 13 race, Heller responded, “Not today.”

At Monday’s swearing-in ceremony in the Senate chamber, Heller was walked down the center aisle by Nevada’s senior senator, Senate Majority Leader Harry Reid (D), who was sporting a black eye and had his right arm in a sling after a running accident last week.

Reid stood alongside Heller as Biden administered the oath. Heller then shook hands with Biden, followed by Reid, who used his uninjured left hand to shake Heller’s.

Among the senators present in the chamber for Heller’s swearing-in were Senate Minority Leader Mitch McConnell (R-Ky.) and Republican Sens. John Cornyn (Texas), Lamar Alexander (Tenn.), Mike Lee (Utah), Jon Kyl (Ariz.) and Roger Wicker (Miss.).

Biden – who often lingers at the Capitol after swearing in new senators to chat for a few minutes with reporters – left a little over 20 minutes after he arrived, telling reporters he had no comment on their questions regarding the White House’s deficit-reduction talks.