WAUKESHA, Wis. — Gov. Scott Walker, who built a national conservative following by crippling public employee unions and then defeating an effort to recall him, announced on Monday that he was running for president as a Washington outsider who would reduce taxes, challenge Iran and Russia, and cut the size of the federal government.

Starting his speech with the words “I love America,” Mr. Walker, a Republican, said the nation’s fiscal health and global reputation required undoing many of President Obama’s priorities, from the Affordable Care Act to any deal with Iran on its nuclear program. He cast himself as an unwavering fighter who would not compromise his principles as president but would rather pursue a decidedly conservative agenda to make people less dependent on government programs and to transfer power from Washington to the states.

“We need new, fresh leadership, leadership with big, bold ideas from outside of Washington,” Mr. Walker said to cheers from several thousand inside the Waukesha County Expo Center. “The kind of leadership that knows how to get things done, like we’ve done here in Wisconsin.”

Mr. Walker’s political message, as well as his national appeal as a Midwestern governor with an Everyman persona, will be tested right away as he campaigns this week in Nevada, South Carolina and New Hampshire, states that hold early and influential contests.