OTTAWA, July 28 (Reuters) - Canada"s Liberal Prime Minister Justin Trudeau is poised to trigger a snap election two years ahead of schedule, betting that high vaccination rates and a post-pandemic rebound will help him prolong and strengthen his grip on power. Trudeau, 49, is eyeing September for what would be his third election, sources said. He first won a majority in 2015. But in a federal vote two years ago, after decades-old blackface pictures surfaced, Trudeau came up short of a majority, forcing him to depend on opposition parties to pass legislation. The Liberals need to pick up just over a dozen more seats in the 338-seat House of Commons to be able to govern on their own. A fall vote would be "an election of inches, not giant steps. We"re looking at 15 seats or so," said Allan Tupper, professor of political science at University of British Columbia at Vancouver. "Failure would be not to win a majority ... and repeat the status quo," Tupper said. If Trudeau were to get only another minority, it would almost certainly lead to questions about whether the party needs a fresh face, analysts said. The official opposition Conservative Party has consistently lagged in polls, suggesting Canadians could reward the Liberals for their handling of the pandemic. There are precedents that look promising for the Liberals. Four of Canada"s 10 provinces held elections during the pandemic with the sitting government winning every time. With a majority in the House of Commons, Trudeau would have a free hand to follow through on his stated policy priorities of fighting climate change and supporting those who suffered most during the pandemic. Pollsters say there is a window this fall in which the Liberals could win a majority, as Canadians embrace the freedom of being vaccinated and the latest budget injects billions of dollars into the economy. Nationally, Liberals would win 34%, compared to 29% for the Conservatives and 22% for the left-leaning New Democrats, pollster Leger said in a survey published last Wednesday. That is probably still short of a majority, but polls by other companies have put the Liberals further ahead. "SWEET SPOT" The timing of the election is important "because the pandemic has been a highlight in the Liberal government"s performance in the eyes of Canadians," said Shachi Kurl, executive director of polling firm Angus Reid Institute. In the first quarter, 45% of adults said the COVID-19 response was by far the top issue on Canadian voters" minds, but in the third quarter it was the ninth issue at 19%, an Angus Reid poll from July 16 shows. "So Justin Trudeau now has to stay in this sweet spot wherein the pandemic is still on the minds of Canadians," Kurl added. COVID-19 case numbers are down dramatically, as are hospitalizations and deaths, and vaccination levels are among the highest in the world. read more The Bank of Canada earlier this month painted an optimistic picture of growth heading in the second half of the year. But another potential source of trouble is that - instead of fading from peoples" minds - COVID-19 will come roaring back as more contagious virus variants spread, as is happening in the United States and elsewhere. read more "The reality is that with the Delta variant and other variants of concern out there, it is likely that we will see a rise in cases over the coming months," Trudeau said on Tuesday. Even if Canada is largely spared, a COVID-19 resurgence elsewhere still could hurt, said Doug Porter, the chief economist at BMO Capital Markets. "There is a risk that markets could become concerned about a fourth wave ... If we had a more serious correction in markets, that could affect sentiment here in Canada as well," he said. If the Liberal leader were to fall short of a majority for a second time, it might be an indication that voters are growing tired of Trudeau, triggering talks of succession. "There are still a number of Canadians who like Justin Trudeau very much, but in terms of that mania, that fever, that passion for Justin Trudeau - that is long gone," Kurl said.