WASHINGTON (Reuters) - Republican presidential candidate Ted Cruz won the backing on Tuesday of Wisconsin Governor Scott Walker, gaining a boost in the contentious three-way race a week before that state’s presidential primary.

Walker, himself a former Republican presidential candidate, emphasized he was not endorsing the U.S. senator from Texas in an effort to thwart Republican front-runner Donald Trump from clinching the party’s nomination for the Nov. 8 election.

“To me, I’m all in,” Walker said in a radio interview on WTMJ radio in Milwaukee. He said he planned to campaign for Cruz throughout the state before the April 5 Wisconsin primary.

Wisconsin polls, though scarce, show Trump and Cruz running neck-and-neck in the state. A recent Emerson poll showed Cruz taking 36 percent and Trump with 35 percent. Ohio Governor John Kasich lagged in third place, with 19 percent.

Walker said he chose to back Cruz because he was a principled constitutional conservative who would stand up to big government special interests and give power to the states.

“I just fundamentally believe if you look at the facts, if you look at the numbers, that Ted Cruz is in the best position by far to both win the nomination of the Republican Party and to then go on and defeat Hillary Clinton in the fall this year,” Walker said.

Walker’s endorsement adds to a number of other more mainstream Republicans stepping up behind Cruz as the best alternative to Trump, who has racked up a strong delegate lead but alienated many party leaders with his harsh views on immigration, Muslims and women.