Barack Obama's legacy now depends on the election of Hillary Clinton
Since before he took office, US President Barack Obama has been compared to Ronald Reagan.
His progressive supporters hoped he would move politics to the left as much as Reagan had moved it to the right.
Mr Obama's last State of the Union address shows that this transformation remains incomplete.
That he has moved public policy in his direction is undeniable. The liberal coalition has expanded, and his party has become correspondingly bolder: his speech showed that too.
The Obama of 2009, making his first address to Congress, felt it necessary to promise to address the "growing costs" of Social Security.
But to leave his mark on American politics for an era, and not just a presidency, Reagan had to do more than just change his own party or even public policy. He had to move the opposition party in his direction, too.
And to do that, he had to be succeeded by an ally.
His successor didn't have to be someone who inspired the new Reagan coalition, as, indeed, George HW Bush didn't.
The successor didn't have to be someone who had always been an ally: Bush had run against Reagan in the 1980 primary, and represented a more moderate tendency in the party.
To help consolidate the victory of Reaganism, Reagan's successor just had to win the election running as a candidate of continuity.
Mr Obama and Hillary Clinton now have the same relationship.
She evidently does not inspire the liberal coalition, as the Democratic-primary polls suggest. Her political career began in a pre-Obama era, when Democrats were more nervous about offending conservative sensibilities than they are today.
But Mr Obama's legacy depends on her, just as Reagan's did on George HW Bush. If she wins, his policy victories are much more likely to be locked in and the Republican Party much more likely to make peace with them.