DENVER—Colorado activists on the right and the left looking to upend the political establishment notched wins this weekend at statewide party assemblies.

In the governor's race, Republican businessman Dan Maes, who has little name recognition or money, captured more delegate votes than the favorite, former congressman Scott McInnis. The tally was close: 1,741 to 1,725.

Mr. McInnis had enjoyed a big lead in the polls—50% to 15% in the most recent survey of GOP voters, released last week by Public Policy Polling.

"The Republican Party needs to wake up and realize that there is a new sunrise," said Michael Schneider, a delegate who supported Mr. Maes.

Republican Ken Buck, a county district attorney seeking his party's U.S. Senate nomination, won 77% of the delegate vote, lifted by support from tea-party activists and South Carolina Sen. Jim DeMint, who recently endorsed him and praised his conservative credentials.

Mr. Buck's main GOP opponent, former Lt. Gov. Jane Norton, who has been considered the front-runner, bypassed the assembly and is trying to get on the August primary ballot by collecting voter signatures. Those petitions are due next week.

Messrs. Maes and Buck "are riding this tide of disaffection that's pushed in some measure by the tea-party folks," said John Straayer, a political scientist at Colorado State University.

Generally, the assemblies are attended by the most energized members of each party, but the nomination fights will be settled in primary elections in August. Candidates with the most delegate votes at the assemblies get top billing on the primary ballot.

Mr. Straayer cautioned that the electorate for the August primary is broader—and perhaps less enamored of insurgents—than the core party activists who gathered Saturday.

The Democratic Colorado assembly also featured a solid victory for challengers, as a former speaker of the state House, Andrew Romanoff, won 60% of the delegate vote for his party's U.S. Senate nomination. The incumbent, Sen. Michael Bennet, won 40%.

Mr. Romanoff cast his victory as an upset of the party establishment, akin to the voting earlier this week that ousted Sen. Arlen Specter in Pennsylvania and forced Sen. Blanche Lincoln of Arkansas into a runoff. Mr. Specter and Ms. Lincoln are both Democrats.

"Today, we joined our counterparts in Arkansas and Pennsylvania with one voice, loud and clear. To the party bosses in the U.S. Senate, we say this: 'Stiffen your spine or step out of the way,' " Mr. Romanoff said.

Mr. Bennet, however, retains a huge lead in fund raising, and many political observers expect him to win the August primary comfortably. Moments after the assembly vote, his campaign manager emailed supporters a plea for more donations—a plea focused more on November's general election than the primary.

"The ground is fertile for Romanoff, said Eric Sondermann, an independent political analyst based in Denver. "This could be one of those uprisings a la some of those other things we've seen around the country where the party really rallies around the insurgent and rejects the incumbent."

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