WASHINGTON — Rep. Ron Paul figured this day would come – but not quite this soon.

Paul stood in the back of the Senate chamber Wednesday with the smile of a proud father as he watched Vice President Joe Biden administer the oath of office and make his son, Rand Paul, the new junior senator from Kentucky.

“It’s pretty significant,” said the elder Paul, R-Texas, who a few hours later was sworn in for a 12th two-year term in the House of Representatives. “I felt pretty good about it. It’s a once-in-a-lifetime experience.”

And a historic one. Their swearing-ins marked the first time in congressional history that a child served in the Senate while the parent simultaneously served in the House, according to the Senate historian’s office.

“It was such an exciting experience,” Rand Paul said of his swearing-in. “We had a lot of family members, all 30 immediate family members. We went to Tony Cheng’s last night; they gave us the whole floor.”

During the 2010 congressional primaries, Rand Paul became one of the main faces of the tea party movement after he knocked off an establishment Republican candidate, Kentucky Secretary of State Trey Grayson, who was backed by Senate Minority Leader Mitch McConnell and former Vice President Dick Cheney.

Much of Wednesday was about pomp and circumstances for Rand and Ron Paul. They were sworn in, attended a joint reception at the U.S. Capitol Visitor Center, and the younger Paul participated in a swearing-in re-enactment ceremony in the Capitol’s old Senate chamber.

But the newly minted senator from Kentucky also tended to some business, firing a warning shot to his own party in a television interview that it better not renege on its promise to implement $100 billion in spending cuts within a year of taking control of the House.

The New York Times reported Wednesday that House Republican leaders are backing away from that vow, outlined in the GOP’s “Pledge to America,” and are looking to slash $50 billion to $60 billion in spending instead.

“The debt problem is enormous,” Rand Paul said on ABC’s “Good Morning America.” “You know we’re bringing in about $2 trillion and spending nearly $4 trillion, so $50 billion is not enough to scratch the surface. … We have to find much, much more. If you go back just to 2008 levels, that’s $100 billion; we have to at least do that.”