FREEPORT — Nirav Shah, the former public health official and defeated Democratic gubernatorial candidate, opened his new Senate campaign by casting himself as the best person to take on Republican Sen. Susan Collins in the wake of Graham Platner’s decision to exit the race.

Shah, who held the No. 2 post at the U.S. Centers for Disease Control and Prevention after guiding Maine’s COVID-19 response at the Maine CDC, presented himself as both a political outsider and an experienced policymaker at a Thursday afternoon news conference in the nearly empty onetime office of his campaign for governor.

The event heralded the start of an extraordinary two-and-a-half-week period of campaigning to replace Platner in one of the country’s most closely watched Senate races.

“This race is a referendum on Susan Collins and her enabling of Donald Trump’s worst instincts and most harmful policies,” Shah said, adding that the Maine Democratic Party’s truncated and still somewhat unclear process for picking a replacement nominee should focus on “which of the candidates is best able to prosecute the case against Senator Collins over the next 117 days.”

He argued that he could earn the nomination with support from Platner’s passionate progressive base after the oysterman-turned–Senate nominee on Wednesday night announced his withdrawal amid a rape allegation he denies. Shah and Platner have strikingly different backgrounds and brands, and Platner backed three other gubernatorial candidates in last month’s primary.

Two of those candidates, former Maine Senate President Troy Jackson and Secretary of State Shenna Bellows, have also entered the Senate race but had not yet held campaign events. Jackson, a northern Maine logger who campaigned with Platner before the June 9 primary election and was similarly endorsed by Sen. Bernie Sanders of Vermont, moved quickly to try to take on Platner’s antiestablishment progressive mantle after his Senate run began to sputter out. (Shah, Jackson and Bellows finished second, third and fourth in the governor’s race, respectively, to Hannah Pingree.)

Surrounded by a hastily assembled group of supporters, Shah spoke to reporters and television cameras and directly addressed voters who backed Platner: “You have an important place in this campaign, and we welcome your voices.”

Asked about his ability to win over progressives, Shah said, “I’m proud of the fact that when it comes to matters of policy, there is very little light between Graham and me.” He mentioned his support for “Medicare for all” and raising taxes on the rich.

He also made the case that he, like Platner, was not a “career politician,” seeking to distinguish himself from the other recently emerged Senate candidates. At the same time, Shah touted his experience in foreign policy, such as CDC work on health diplomacy, as a “big differentiator relative to my other colleagues in the race.”

Shah previewed how he would attack Collins, a five-term incumbent who is the only Republican senator seeking reelection this year in a state Trump lost two years ago, saying she had not improved Mainers’ lives over nearly three decades in the Senate.

“She touts the dollars that she has brought to Maine, but when you look at the math, it becomes clear that Maine loses more than we bring in because of Susan Collins,” Shah said.

Before the news conference began, a group of attendees worked on updating Shah yard signs by taping over the word “GOVERNOR” and handwriting “SENATOR” on top. He entered the room at 2 p.m. to raucous applause.

Erin Evans, 55, a Portland resident and former neighborhood organizer for Platner’s campaign, took the podium first to voice her support for Shah.

“The people who have been working so hard in Maine since last summer to defeat Susan Collins are not defeated by what has happened,” she said. “This U.S. Senate campaign was never built on one person or for one person.”

How many Platner supporters, and critically how much of his army of organizers and volunteers, Evans represents in embracing Shah remains to be seen.

David Tarbet, 84, a retired lawyer in Topsham who volunteered for Shah’s gubernatorial campaign and stood behind him on Thursday, said he believed Shah could pick up support from progressives by emphasizing his policy views. When it came to Shah’s argument that he could follow Platner as a fellow outsider, Tarbet said, they are like “apples and oranges.”

The state Democratic Party is planning a convention with about 600 delegates to determine its next Senate nominee by the July 27 deadline, but by Thursday afternoon it had not announced many details about the process.

Shah said he was eager for his supporters to become delegates in a transparent and fair way.

“If we end up with a higher number of delegates who are Shah supporters, that should only be because there are more Shah supporters in Maine, not because we’ve gamed the rules,” he said.