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A blend of the nouns hope and opium, the noun hopium—also, and originally, hopeium—designates a notional drug supposed to have been ingested by a person who maintains an unrealistically optimistic outlook.

—Note: The noun hopium (as a blend of hope and opium) must be distinguished from hopium, a rendering of a non-standard pronunciation of the noun opium. Such a rendering occurs, for example, in the following from Peregrinations of Pickwick; A Drama, in Three Acts (London: Published by W. Strange, 1837), by William Leman Rede [Act 3, scene 2, page 31]:

Sam. Vell, you young hopium eater!
[…]
Sam. A werry hinteresting idea.

Since 2008, the noun hopium, also hopeium, has often been used in reference to the Barack Obama Hope poster and associated political ideas. This image of the U.S. Democrat statesman Barack Obama (born 1961), 44^th President of the USA from 2009 to 2017, came to represent his 2008 presidential campaign. The following, for example, is from Residents hope for Exit 408, by Greg Johnson, published in the News-Sentinel (Knoxville, Tennessee, USA) of Wednesday 27^th January 2010 [page A4, column 1]:

As America discovers the dangers of “Hopeium”—the devious drug peddled by President Barack Obama in his 2008 campaign—some are still seeking the stuff in Sevier County. Hopeium in this case comes from Hopemeisters who are pushing the idea that a new exit off Interstate 40 to alleviate some of Sevier’s severe traffic troubles is imminent.
[…]
[…] Democrats in Congress have much bigger issues than the Exit 408, but Hopeium is addictive.

These are, in chronological order, the earliest occurrences of the noun hopium, also hopeium, that I have found:

1-: From Fun (London, England) of Saturday 6^th February 1864 [page 212, column 1]:

The Best Drug in the Pharmacopœia.—Hope-ium!
We Think So.—Might not a “Life” of Louis the Eleventh appropriately commence with “onze upon a time?”

2-: From the column The Week, published in The Tomahawk. A Saturday Journal of Satire (London, England) of Saturday 28^th May 1870 [page 208, column 1]:

It is possible that Sir Wilfrid Lawson may head a great Ante-opiate Movement, the object of which will be to get all people afflicted with toothache to pledge themselves to abstain from laudanum. It will, of course, be marshalled under the title of The Band of Hope-ium!

3-: From the column Local Laconics, published in The Yonkers Gazette (Yonkers, New York, USA) of Saturday 13^th December 1873 [page 3, column 2]:

Who is there that does not deal in “hope-ium” these hard times?

—Note: Here, the form hope-ium seems to correspond to an end-of-line splitting of hopeium—it was the latter form that appeared in reprints of that line, for example in the Champaign County Gazette (Champaign, Illinois, USA) of Wednesday 24^th December 1873 [page 1, column 1].

4-: From the column Varieties, published in the Taunton Courier, and Western Advertiser (Taunton, Somerset, England) of Wednesday 7^th May 1884 [page 3, column 1]:

According to Dr. Mortimer Granville there is no drug in the pharmac hope-ia half so powerful as hope. ’Ope-ium even comes in second.

5-: From the title given to a letter published in the column Home Town News, by Frank Good, published in The Wichita Eagle and The Beacon (Wichita, Kansas, USA) of Sunday 24^th June 1973 [page 7A, column 3]:

DESPITE WHATEVER YOU MAY BE HOPE-IUM,
THESE POPPIES DON’T MAKE OPIUM!

“Dear Frank, I am looking for a source for oriental poppy pods—not for opium, as you might suspect—but to use in decorative wreaths and dried flower arrangements,” notes Mrs. P. L. Howell [&c.].

6-: From Salvation and Suicide: An Interpretation of Jim Jones, the Peoples Temple, and Jonestown (Bloomington and Indianapolis: Indiana University Press, 1988), by the U.S.-born South-African professor of religious studies David Chidester (born 1952) [2 The Classification of Persons, 2.1 Superhuman, page 57]:

“What is perfect love?” Jones asked. The answer was “Socialism, Apostolic Socialism, as it was every time the Holy Spirit descended in the New Testament, they sold their possessions” (Q967). This was the practical God in distinction to Sky Gods, Buzzard Gods, and the unknown God worshipped by those who were addicted to the “hopium” of myth. It was a God with practical consequences for the reordering of human relations, the elimination of private property, and the sharing of human and material resources.