Mr. Cruz, 44, who was elected in 2012, gained fame by trying to block funding for President Obama’s health law in 2013. His tactics resulted in a partial closure of the federal government, a turn of events that made “Cruz” a four-letter word to his party’s leadership but endeared him to many conservative activists hungry for politicians who would confront both parties.

But in recent months, Mr. Cruz has been overshadowed by other potential Republican candidates in the early competition for donors, staff, volunteers and news coverage. Most notably, Mr. Walker has drawn attention from those interested in an alternative to former Gov. Jeb Bush of Florida.

In multiple meetings since January, Mr. Cruz’s advisers discussed Mr. Walker’s effort to win support from both the center-right and more conservative wings of the party, according to Republicans familiar with the sessions but who were not authorized to speak on the record.

Witnessing Mr. Walker’s early boomlet, along with some of the Wisconsin governor’s initial stumbles, prompted Mr. Cruz to announce early, ahead of the other hopefuls in both parties, the Republicans briefed on his strategy said.

Mr. Cruz and his advisers, recalling his path to victory in Texas, saw more opportunity than risk in dispensing with any exploratory phase: In 2012, Mr. Cruz entered what was initially a crowded Republican field more than a year before the Senate primary and slowly earned support from conservative activists through intensive travel and on the strength — and uncompromising nature — of his rhetoric.

He now plans on pursuing a similar presidential campaign, portraying himself as not only the most doctrinaire candidate, but as the one most willing to fight for the principles of the conservative movement.

Kellyanne Conway, a Republican pollster, said that distinction could help Mr. Cruz win over the party’s hard-line activists.