INDIANAPOLIS — Republican U.S. Senate candidate Todd Young ratcheted up his rhetoric against former Democratic Sen. Evan Bayh on Tuesday after receiving a big boost in outside campaign spending.

The U.S. Chamber of Commerce announced it plans to spend at least $1 million on Indiana television advertisements supporting Young after spending that amount to promote him during the Republican primary.

The chamber’s endorsement of Young comes even though the Washington-based business group selected Bayh soon after he left the Senate in 2011 to speak at events around the country advocating more “checks and balances to the regulatory process.”

Leaders of the national and state chambers joined Young at a news conference Tuesday where he accused Bayh, also a former two-term Indiana governor, of being a threat to the “American free enterprise system.” Young also personally attacked Bayh, calling him a “fundamentally flawed” person.

“His partisanship transcends the rote Republican versus Democrat dynamic,” Young said. “Evan Bayh looks out for himself.”

Bayh spokesman Ben Ray said the attacks show the Young campaign is desperate.

The attack on Bayh is “pretty rich” considering Young took donations from Carrier Corp., which is moving more than 1,000 jobs from Indiana factories to Mexico, Ray said.

Bayh’s surprise decision last month to enter the race for the Senate seat he gave up in 2010 after two terms has forced national Republican groups to invest heavily in Indiana in hopes of keeping the seat now held by retiring GOP Sen. Dan Coats.

Young, a three-term congressman from Bloomington, has needed that boost as he trailed Bayh in name recognition and had about $1.2 million campaign cash on June 30 — far behind the nearly $9.5 million Bayh had at the time.

About $2.8 million has been spent so far on Young’s behalf by groups such as the National Republican Senatorial Committee; the Senate Leadership Fund, which is tied to Senate Majority Leader Mitch McConnell; and Freedom Partners, a group steered by billionaire industrialists Charles and David Koch, according to Federal Election Commission reports.

The Indiana Chamber of Commerce said it reviewed its decision to endorse Young for the Senate seat after Bayh’s entry into the race. The U.S. chamber gives Young a 91 percent lifetime congressional vote rating, compared with Bayh’s 55 percent.

Indiana Chamber Vice President Caryl Auslander said the group picked Young because of his support for tax reforms backed by the organization and his sponsorship of a bill that would require Congress to vote on major federal regulations before they go into effect.