Republican

The senator from Texas had been seen as the Republican party's only viable alternative to Mr Trump.

"We left it all in the field in Indiana, and the voters chose another path," he told supporters in Indiana.

"With a heavy heart, but with boundless optimism for the long-term future of our nation, we are suspending our campaign."

Trump's plan for an overhaul of the U.S. tax code would eliminate federal income taxes on individuals earning less than $25,000 and married couples earning less than $50,000. He said those individuals would receive "a new one page form to send the IRS saying, 'I win.'"

Trump's plan would also disproportionally benefit wealthy earners by lowering the highest income tax rate from 39.6 percent to 25 percent, and businesses by cutting their tax rate — from major corporations to mom-and-pop shops — to no more than 15 percent.

The Wisconsin governor planned a news conference for Monday evening in Madison, where he was expected to become the second major GOP candidate to quit the race.

The people spoke on condition of anonymity because they were not authorized to speak before Walker's announcement.

Walker tried to capitalize on his record taking on unions and got off to a strong start after a well-received speech to Iowa conservatives in January, formally launching his campaign in July.