His conservative tendencies on the Slavery question, however, were not in harmony with the demands of public opinion in Massachusetts, and in 1851 he was defeated for the governorship by George S.Boutwell, and for the senatorship by Charles Sumner.
Mr.Winthrop's political career closed when he was forty-two years of age.
WHIGS ABANDON THE WILMOT PROVISO.
The events of the year 1847 had persuaded the Whig leaders that, if they persisted in the policy embodied in the Wilmot Proviso, they would surrender all power to control the ensuing Presidential election.

By clever management and the avoidance of issues which involved the slavery question, they felt reasonably sure of the votes of Delaware, Maryland, North Carolina, Kentucky, and Tennessee, with a probability of securing Georgia, Louisiana, and Florida.
To throw these States away by an anti-slavery crusade was to accept inevitable defeat, and disband the Whig party.

Mr.Winthrop was therefore representing the prevailing wishes of Northern Whigs when he used his influence to restrain rather than promote the development of the anti-slavery policy which had been initiated with such vigor.
The result of this change was soon visible.

In the preceding House, with a large Democratic majority, the Wilmot Proviso had been adopted.

In the Whig House, over which Mr.Winthrop presided, it was found impossible to repeat the vote during the preparations for the national contest then impending.