Historically, the United States economy did better on average when Democrats were US President than when Republicans were US President. This was certainly true over the last several decades. The reasons for this are uncertain and debated. Ten of the 11 US recessions between 1953 and 2020 began under Republican US Presidents.

Job creation
Job creation refers to the number of net jobs added, which the Bureau of Labor Statistics reports on a monthly basis.  
Journalist Glenn Kessler of The Washington Post summarized the total job creation by US President. For the 13 US Presidents starting from Harry Truman, total job creation was about 70.5 million for the 7 Democratic US Presidents and 29.1 million for the 6 Republican US Presidents. The Democratic US Presidents were in office for a total of 429 months, with 164,000 jobs per month added on average, while the Republican US Presidents were in office for 475 months, with a 61,000 jobs added per month average.  This monthly average rate was 2.4 times faster under Democratic US Presidents.

Former President Bill Clinton said in 2012 that since 1961, Democratic US Presidents produced 42 million jobs in 24 years while Republicans produced 24 million jobs in 28 years.

Economists Alan Blinder and Mark Watson said that, for the 1949-2012 time period, annual job growth was 2.6% for Democratic US Presidents and 1.2% for Republican US Presidents.

Job creation by US Presidency

GDP growth

GDP is a measure of both economic production and income. The Economist reported in August 2014 that real (inflation-adjusted) GDP growth was about 1.8% greater under Democratic US Presidents, from Truman up to Obama's first term, which ended in January 2013. Blinder and Watson calculated the average Democratic real GDP growth rate as 4.3%, vs. 2.5% for Republicans, from President Truman's elected term up to President Obama's first term, which ended January 2013. CNN said in September 2020 that annual average GDP growth was 4.1% under Democratic US Presidents and 2.5% under Republican US Presidents.

The New York Times said in February 2021 that since 1933, annual average GDP growth was 4.6% under Democratic US Presidents and 2.4% under Republican US Presidents.

Unemployment rates
Blinder and Watson found that the US unemployment rate fell under Democratic Presidents by an average of 0.8%, while it increased under Republican Presidents by an average of 1.1%.  Journalist Andrew Soergel wrote in U.S. News & World Report in October 2015 that ever since John F. Kennedy's Presidency, the unemployment rate was lower at the end of every Democrat's Presidency than at the start of it whereas this is only true for Ronald Reagan among Republican US Presidents during the same time period. (Soergel's statement here is slightly wrong since the US unemployment rate was exactly the same–at 7.5%–at both the start and the end of Democrat Jimmy Carter's Presidency.)

Historical unemployment rate statistics

Budget deficit
Blinder and Watson reported that budget deficits were smaller under Democratic US Presidents, at 2.1% potential GDP versus 2.8% potential GDP for Republican US Presidents, a difference of about 0.7%. They wrote that higher budget deficits should in theory have boosted the economy more for Republicans, and therefore cannot explain the greater GDP growth under Democrats.

Stock market returns
Stock market returns are also higher under Democratic US Presidents ever since 1945, according to both CNN and Blinder and Watson.

Recessions
CNN reported in October 2020 that 10 of the last 11 recessions began under Republican US Presidents, and that “Every Republican president since Benjamin Harrison, who was in office from 1889 to 1893, had a recession start in their first term in office.” NBER reports the start date of recessions; the following list includes the President in office at that time and their party:
February 2020 (Trump / R)
December 2007 (Bush 43 / R)
March 2001 (Bush 43 / R)
July 1990 (Bush 41 / R)
July 1981 (Reagan / R)
January 1980 (Carter / D)
November 1973 (Nixon / R)
December 1969 (Nixon / R)
April 1960 (Eisenhower / R)
August 1957 (Eisenhower / R)
July 1953 (Eisenhower / R)
November 1948 (Truman / D)

Blinder and Watson calculated that the US economy was in recession for 49 quarters from 1949-2013; 8 of these quarters were under Democratic US Presidents and 41 of these quarters were under Republican US Presidents.

Commentary
During a March 2004 interview, future Republican US President Donald Trump stated: "It just seems that the economy does better under the Democrats than the Republicans."

Journalist David Leonhardt wrote in the New York Times that economists he interviewed were unsure about the causes of the better economic performance under Democratic US Presidents. However, one proposed cause was that Democratic US Presidents were, on average, more pragmatic than Republican US Presidents were. Random good luck and the condition of the US economy at the start of US Presidents' terms have also been suggested as possible causes of this due to the fact that it's easier for an economy to grow rapidly and create a lot of jobs right after a recession as opposed to after a very long period of economic growth and strong job creation.

Statistics
The table below compares various economic data for Democratic and Republican US Presidents, using the Blinder & Watson data (typically Truman's elected term through Obama's first term).

References

Economic history
History of the United States