U.S. consumer sentiment improved more than expected in December, rising to the upper end of the favorable range it has traveled since the beginning of 2017, preliminary results from the monthly University of Michigan Surveys of Consumers showed.

The consumer sentiment index climbed to 99.2 in December, up from 96.8 in November, with the index's average for the past three years reaching the highest sustained level since President Bill Clinton's administration, which holds the record.

The consensus estimate from economists polled by Econoday was for an index reading of 96.9 in December.

The current economic conditions index rose month over month to 115.2 from 111.6, while the consumer expectations index improved to 88.9 from 87.3.

Surveys of Consumers Chief Economist Richard Curtin attributed most of the early December increase to upper-income households, which reported near-record increases in household wealth, primarily due to higher stock prices. "These gains were aided by declining inflation expectations, with long term inflation expectations returning to an all-time low," Curtin said in a statement.

Despite the widely covered impeachment process against President Donald Trump, only 1% of survey respondents cited the issue in their responses to questions.

However, the data showed the increasing role partisanship is playing in economic expectations.

"The average gap between Democrats and Republicans was 18.7 points in the Obama administration and 41.6 points since Trump took office," Curtin said. "Importantly, the views of Independents closely track the overall Sentiment Index, with a mean of 96.6 versus 97.0 for all consumers."