Updated: 11:56 p.m.

Appointed Colorado Sen. Michael Bennet beat back a spirited Democratic primary challenge from former state Speaker Andrew Romanoff on Tuesday and can now look forward to a tough general election this fall in a key Senate battleground state.

Bennet, who received 54 percent of the vote to 46 percent for Romanoff, will face Weld County District Attorney Ken Buck in what is expected to be a top contest in November. Buck held off national party favorite Jane Norton on Tuesday night to secure the GOP nomination.

Bennet’s 8-point victory margin was larger than what many race watchers were expecting heading into Tuesday after a late surge by Romanoff had some wondering whether Bennet, despite support from the White House and the Democratic Senatorial Campaign Committee, might be the next incumbent to fall in a primary this cycle.

During the first 10 months of the race, it didn’t appear that the well-funded Bennet would have much trouble with Romanoff, even though Romanoff had better connections in state Democratic circles stemming from his eight years in the state legislature. But Romanoff gained traction late in the race by criticizing Bennet for taking more than $1 million in campaign contributions from special interest groups and for being the choice of the national party establishment. A Denver Post poll from late July showed Romanoff ahead by 3 points.

In the end, Bennet likely benefited from Colorado’s mostly mail-in primary system, which allowed him to better leverage his large cash advantage. Forty-six of Colorado’s 64 counties held only mail-in elections this year, and ballots began to be mailed to voters July 19. Bennet, who outspent Romanoff by more than $4 million during the primary, flooded the airwaves in the lead-up and during the three-week period in which voters could send in their ballots. Romanoff, meanwhile, was forced to mortgage his house to find enough money to stay on the air during the final stretch.

The primary system meant that late-breaking developments were less likely to have an effect than they would under the more standard Election Day format. On Friday, the New York Times published a story that was critical of Bennet for a financial deal he helped put together during his time as head of the Denver Public Schools. Romanoff jumped on the story, but by the time it was published about a quarter of a million Democratic ballots had already been cast.

Buck, who built his grass-roots campaign on the back of tea party activists, edged Norton 52 percent to 48 percent in the Republican race.

Although national GOP leaders such as Sen. John McCain (Ariz.) and the National Republican Senatorial Committee were pulling for Norton in the primary, it’s a sure bet that the party establishment will quickly rally around Buck in a race that is being viewed as one of the most competitive of the cycle.

Some Democratic strategists have been openly pulling for Buck because they believe his tea party connections make it easier to paint him as out of the mainstream in the general election. But a survey by a Democratic polling firm from over the weekend found Buck might be the tougher candidate against Bennet in November, if only slightly. The survey by Public Policy Polling showed Bennet ahead of Norton by 6 points, while Buck and the Senator were in a statistical tie.

Although Buck was carried to victory Tuesday night by his support among those identified with the tea party, he wasn’t always the darling of the anti-establishment crowd. Last fall, five months after he joined the race, Buck’s campaign seemed dead. Norton, a former state Public Health and Environment Department director and state Representative, had entered the race in September to great fanfare within party circles, and Buck saw his fundraising dry up.

But as Norton came to be seen as the establishment candidate, tea party activists began casting about for an alternative, and they found Buck. An endorsement from tea party champion Sen. Jim DeMint (R-S.C.) helped Buck open new fundraising channels, and his campaign started gaining momentum during the spring and summer.

By May, Norton had decided to skip the state GOP’s assembly and secured her place on the primary ballot by gathering petitions. That move was widely seen as a way to avoid the embarrassment of coming in second to Buck among the more hard-core activists who make up the delegate population.

But after Buck appeared to take a commanding lead in polling in June, the Norton campaign seemed to find its stride in July. Norton used her $1.6 million financial advantage to blast Buck for a series of gaffes, including one in which he appeared to slam the very tea party supporters who helped his campaign catch fire.

Norton closed the gap, but in the end, it wasn’t enough.