The federal agency in charge of deportations is conducting a far-reaching training course to push immigration enforcement officers and prosecutors nationwide to focus their efforts on removing immigrants convicted of crimes.

The training course is the clearest sign yet that administration officials want to transform the way immigration officers work, asking them to make nuanced decisions to speed deportations of high-risk offenders while halting those of illegal immigrants with clean records and strong ties to the country. The policy is President Obama’s most ambitious immigration initiative before the November elections, senior administration officials said.