The phrase "underscore the likelihood" is correct and usable in written English.
You can use the phrase to emphasize the probability of something happening, or to highlight the fact that the likelihood of something is important. For example: Recent findings underscore the likelihood that climate change will bring increasingly intense storms in the coming years.
The new goals underscore the likelihood that the 2012 presidential campaign could shatter records.
These sizable effects underscore the likelihood that some combination of passive and active representation at the classroom level affects socially, academically, and developmentally important student outcomes".
First results underscore the likelihood of our assumption.
Our results underscore the likelihood of stem cell-like resistant cells in tamoxifen resistance.
These two studies underscore the likelihood that the perceptions reported in this study are realistic.
These concepts underscore the likelihood of related and overlapping mechanisms contributing to eosinophilia in ECRS.
In the amended filing, Google underscored the likelihood that its executives and employees would sell shares of the company's stock during the initial public offering.
And however persuasive Carney may be with his colleagues, the turbulence of recent weeks has underscored the likelihood that events far beyond his control will take over.
The rise in diagnoses was also generally greater among 14- to 17-year-olds, underscoring the likelihood that these were previously unrecognized cases.
David C. Vladeck, the lawyer who argued for the release of the transcripts, said the Greenglass testimony "underscores the likelihood that the testimony used to convict Ethel Rosenberg was, as David Greenglass later admitted, cooked up right before trial.
-- Former Labor Secretary Tom Perez told Politico's Gabe Debenedetti that he has 180 of the 224 votes he needs to become the next DNC chairman – underscoring the likelihood that the vote will go to multiple rounds of balloting.
